1. Field of Invention
The present invention relates to a method for making membrane fuel cell (MFC) electrodes using carbon nanomaterial (CNM)-supported catalysts and, more particularly, to a method for making MFC electrodes by low-voltage electrophoretic deposition (EPD) of CNM-supported catalysts with even and thin electrode catalyst layers.
2. Related Prior Art
In general, MFCs include direct methanol fuel cells (DMFC) and proton exchange membrane fuel cells (PEMFC). An MFC is an electrochemical power generating device incorporated with a proton exchange membrane as the solid polymer electrolyte. Such an acid-type MFC needs platinum-based catalysts with high corrosion resistances and enhanced electrocatalytic activities, e.g., platinum (Pt) and platinum-ruthenium (Pt—Ru), to ensure good performance and long service life. MFCs are able to provide high energy densities, high energy-conversion efficiencies, structural simplicity, excellent durability and portability; therefore, such fuel cells are actively being developed for uses in electric vehicles, laptop computers, mobile phones and other electronic products to replace conventional batteries.
The catalysts of an MFC are generally existing in forms of nano-sized particles, commonly called as nanocatalysts, evenly spread and attached to a conductive carbon black powder, such as Vulcan XC72 (Cabot). The conductive carbon black powder is used as a support for the catalyst with good ability to transfer electrons or currents. Generally, a thin catalyst layer is fabricated on the electrode surface using various methods. For instance, the catalyst can be directly coated on an ion exchange membrane, pre-coated on a separable base and then decaled onto the ion exchange membrane, or pre-coated on a substrate and then hot-pressed onto the ion exchange membrane. In addition, conventional ways to apply the catalysts include brushing, spraying and mesh printing. The objective is to provide an even and thin catalyst layer so as to increase the efficiency and reduce the resistance of the electrode.
With the advancement of nanotechnology, today several carbon nanomaterials (CNMs), e.g., carbon nanotube (CNT), carbon nanofiber (CNF), carbon nanocoil (CNC) and carbon nanohorn (CNH), are commercially available for uses in a variety of new applications. Due to their distinguished characteristics, including excellent electrical conductivities, high material purities and high electrochemical stabilities, these CNMs have been investigated as new catalyst supports, replacing carbon black powders, for the MFC nanocatalysts with an attempt to significantly improve the performances of MFCs. However, the CNMs are generally bulky in structures and much larger than the conductive carbon black powders in dimensions. If the catalysts are supported on such CNMs via a conventional coating process, the resultant catalyst layers are often too thick and slack. In addition, the high consumption of ionomer will cause high resistance in the catalyst layer and, in turn, give rise to poor cell performance. This problem is especially pronounced when a high catalyst loading, such as 2-4 mg Pt—Ru/cm2, is generally required for a DMFC. Hence, the nanocatalysts supported on the CNMs so far cannot adequately be used in a MFC. The performance of a resultant membrane electrode assembly (MEA) is generally poor.
Therefore, the present invention is intended to obviate or, at least, alleviate the problems encountered in prior art.